Optimize math operations in hot code paths #1153

Xorlent · 2026-03-07T03:12:56Z

Xorlent
Mar 7, 2026

Hi Gregg,
What do you think of bitwise operations and shifting in hot areas of code like the encryption and decryption code? Here are a few examples I am looking at:

HAP.cpp

  if(hapClient!=NULL){
    if(!hapClient->cPair){                        // if not encrypted 
      hapClient->client.write(buffer,num);        // transmit data buffer
      
    } else {                                      // if encrypted
      
      encBuf[0]=num%256;                          // *****  BECOMES -> encBuf[0]=num & 0xFF;  // store number of bytes
      encBuf[1]=num/256;                           // *****  BECOMES -> encBuf[1]=num >> 8;  // store high byte
      crypto_aead_chacha20poly1305_ietf_encrypt(encBuf+2,NULL,(uint8_t *)buffer,num,encBuf,2,NULL,hapClient->a2cNonce.get(),hapClient->a2cKey);   // encrypt buffer with AAD prepended and authentication tag appended
      
      hapClient->client.write(encBuf,num+18);     // transmit encrypted frame
      hapClient->a2cNonce.inc();                  // increment nonce
    }
    delay(1);
  }

int HAPClient::receiveEncrypted(uint8_t *httpBuf, int messageSize){

  uint8_t aad[2];
  int nBytes=0;

  while(client.read(aad,2)==2){    // read initial 2-byte AAD record

    int n=aad[0]+aad[1]*256;            // ***** BECOMES -> int n=aad[0]+(aad[1]<<8);  // compute number of expected decrypted bytes

    if(nBytes+n>messageSize){      // exceeded maximum number of bytes allowed in plaintext message
      LOG0("\n\n*** ERROR:  Decrypted message of %d bytes exceeded maximum expected message length of %d bytes\n\n",nBytes+n,messageSize);
      return(0);
      }

Xorlent · 2026-03-07T15:24:59Z

Xorlent
Mar 7, 2026
Author

Do you think the compiler will automatically optimize these operations anyway?

0 replies

Xorlent · 2026-03-08T21:28:46Z

Xorlent
Mar 8, 2026
Author

I ran some statements through the Godbolt.org GCC emulator using -Os:
Division and multiplication looks good, but the generated assembly for (encBuf[0]=num%256;) has extra code to handle negative values. It looks like the optimization for num%256 would help, since num needs to remain a signed value.

0 replies

HomeSpan · 2026-04-03T15:07:34Z

HomeSpan
Apr 3, 2026
Maintainer

Thanks for the above. Given that HomeKit and HAP do not require extreme speeds, I don't think such optimizations would do anything noticeable to performance or the system itself. And sometimes such optimizations make the code less readable (or obvious).

0 replies

Xorlent · 2026-04-03T16:21:05Z

Xorlent
Apr 3, 2026
Author

Understood and thanks as always for your hard work on this project!

0 replies

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